Electric power which has been generated by an off-shore wind farm or a cluster of wind turbines is transmitted to an on-shore utility grid by either AC or DC transmission lines. For shorter distances typically AC transmission is used but AC cables are affected by capacitive charging which limits the length that can be realistically used to about maybe 100 km.
From the publication “Grid Connection and Remote Control for the Horns Rev 150 MW Offshore Wind Farm in Denmark” by Peter Christiansen et al. it is known that the individual wind turbines of a wind farm can be arranged in a plurality of rows, wherein each row consists of a number of serial connected wind turbines. The rows are interconnected at an off-shore transformer sub-station comprising busbars, a transformer and circuit breakers. The off-shore transformer sub-station is positioned on a separate foundation.
Electric power from the individual wind turbines is produced at some specific generator voltage which according to the above-identified publication is transformed in each wind turbine to an intermediate AC voltage level for the transmission of the generated electric power to an off-shore transformer sub-station. Intermediate AC voltage level transmission lines can interconnect the corresponding individual wind turbines in each row and can connect the rows to the off-shore transformer sub-station. The basic task for the off-shore transformer sub-station is to transform the AC power from the concatenated wind turbines from the intermediate AC voltage level to at high AC voltage level. The intermediate AC voltage level may be e.g. 24 kV, the high AC voltage level may be e.g. 150 kV AC.
EP 1 071 883 B1 discloses a wind turbine having a generator, a tower, a foundation on which the tower is provided, and a transformer, which is connected to the generator in order to transfer the electrical power that is produced to a utility grid. The weights of the tower, the generator and the transformer are borne solely by the foundation of the wind turbine. The transformer is arranged on a platform which is attached to the outside of the tower.
EP 1 240 426 B1 discloses an off-shore wind turbine having a tower, a rotor head, at least one rotor blade and an electric subsystems such as switching equipment and/or a transformer, which are housed in at least one container positioned interchangeably at the outside of the tower of the wind turbine. The container is suspended from a supporting framework structure fitted to the tower. This framework structure contains a cable system for lowering and replacing the container.
WO 2008/039121 A1 discloses a wind farm having a plurality of off-shore wind turbines distributed over an area. Each off-shore wind turbine comprises a generator and a first step-up transformer for raising the voltage of the electric power signal generated by the generator. Each off-shore wind turbine is connected, by means of cables, to a further second step-up transformer connected to a main cable in common to all off-shore wind turbines for transfer of the total power generated by the wind farm to a utility grid being located at a considerable distance to said area. The off-shore wind turbines are interconnected in groups. Each group is connected to a separate second step-up transformer located in the region of the group. Further, each second step-up transformer is connected to the main cable in common to all the off-shore wind turbines. Each one of the second step-up transformers may be arranged in one of the wind turbines belonging to the respective group. The second step-up transformer may be arranged on a column structure of the respective wind turbine or on the foundation supporting this column structure.
However, when installing the second step-up transformer at the respective wind turbine the installation effort is serious. In particular, the electric and mechanic installation of the step-up transformer together with necessary electric equipment being external to the step-up transformer might cause a lot of trouble especially when considering rough environmental conditions at an off-shore location.
There may be a need for improving the installation of a step-up transformer at a wind turbine.